The genus Orthopoxvirus (OPV) includes, among others, the human pathogen variola virus (VARV), the zoonotic viruses monkeypox (MPXV) and cowpox virus (CPXV)~ the vaccine species vaccinia virus (VAC) which is also responsible for zoonoses in Brazil and India, and the mouse pathogen Ectromelia virus (ECTV) that causes mousepox in mice and is an outstanding model for human smallpox. All OPVs encode a highly conserved (~87% identical residues) Type 1 interferon (T1-IFN) binding protein (herein T1-IFN bp) that is secreted from infected cells to act as a decoy receptor for T1-IFN in the extracellular milieu and at the cell surface. In this way, the T1-IFNbp suppresses the anti-viral activity of T1-IFN. Using ECTV as a model, we previously showed that T1-IFN bp is essential for OPV virulence and an effective target for vaccination. New preliminary results show that an anti-T1-IFNbp monoclonal antibody (mAb) that inhibits the binding of T1-IFN bp to T1-IFN (but not a non-inhibitory mAb) can cure lethal mousepox when given as late as 5 days post infection (dpi). This suggests the new and exciting possibility that disruption of 1-IFNbp-T1-IFN binding could be an effective treatment for human OPV infections. However, treatment with mouse mAbs is not ideal because they are expensive and can cause an undesirable immune response in humans. Small molecule pharmacological inhibitors of T1-IFN bp would be a much better approach. Hence, we will screen for small molecules that inhibit T1-IFN bp-T1-IFN (Aim 1) and test them for their ability to cure mousepox, an outstanding small animal model for OPV disease (Aim 2). If successful, these drug candidates could be tested in additional OPV animal models as surrogates of human OPV disease. Developing a new simple pharmacological agent against OPVs should have an impact for human health in areas of endemic and zoonotic MPXV, CPXV, VACV and buffalopox virus and may be of extreme importance in case of emerging OPVs or bioterrorist attacks. PUBLIC HEALTH RELEVANCE: The proposed studies are highly significant because they will identify drug candidates for the treatment of poxvirus diseases at late stages of infection. The proposed work is of significance to human health because poxviruses cause endemic diseases in India, Africa and Brazil, are possible emerging pathogens, and there is a risk they could be used as agents of bioterrorism.